A transmission line is generally realized to be able to transport a certain amount of power from a power generating plant to a utility grid. For example, a transmission line can be realized to transport the installed capacity of a power plant, i.e. the maximum or “installed” power that can be delivered by the power plant. For example, a wind farm with twenty 3 MW turbines has an installed capacity of 60 MW, i.e. that wind park can deliver up to 60 MW of active power, and a transmission line connecting the wind park to a power network or “grid” will be designed to be able to transport at least this quantity of power.
Problems arise when a transmission line—for whatever reason—cannot transport the installed capacity. For example, when one transmission line of a double transmission line is out of service, the maximum power transfer capacity of the double line is reduced due to an alteration in the short circuit ratio (SCR) of the system. In such a situation, the double line can no longer carry the installed capacity of a power plant to which it is connected. In the case of a wind power plant, this situation can lead to instability at turbine level, resulting in sustained or poorly damped oscillations seen in voltage, power, current and etc. The maximum power output of the wind park is therefore determined by the limited capacity of the transmission line.
When a transmission line fails or is out of service, the grid will be weakened as a result. The condition of a grid can be quantified in several ways. One way of expressing “grid condition” is by the short circuit ratio of the grid. This is obtained by determining the short circuit capacity of the grid, i.e. the amount of power at a point in the grid in the event of a short circuit. The short circuit capacity depends on the grid impedance at that point, the rated voltage, and any load on the grid. The wind turbines of a wind park can be regarded as an active load on the grid. A short circuit ratio (SCR) for a wind plant can therefore be determined by the grid three-phase short circuit capacity (SCC) divided by the rated power of the wind turbines feeding into the grid. The value of SCR that classifies a grid as “weak” may depend on the network operators. For example, according to the VDN Transmission code 2007 (Verband der Netzbetreiber, Version 1.1, August 2007) a grid is considered “weak” when its SCR drops below 4.0. Generally, a weak grid is characterized by a low SCR and high grid impedance. The issue is exacerbated at lower SCR values, when the system becomes increasingly “weak” with respect to the wind plant, the impedance increasingly high. Generally, an SCR below about 2.5 is considered to approach the limits of modern wind plants to sustain operation. At very low levels of SCR (e.g., below 2.5), any small perturbation or disturbance may cause one or more wind turbines to operate at undesirable operating points, and these turbines may be unable to return to normal operation. As a result, the turbines may trip and disconnect from the system. It is generally held that practical operation of a wind park with an SCR less than 1.25 is challenging, and operation of a wind park with an SCR less than about 0.9 is probably not possible using existing technology without active power curtailment.
Prior methods of dealing with such situations generally involve reducing or curtailing the active power output of the wind turbines of a wind park in order to ensure that the wind turbines continue to operate in a stable manner. In these approaches, a constant amount of active power is curtailed, based on a “worst case” design. However, this means that the power output of the wind park is generally reduced by more than is strictly necessary, and this in turn results in an unnecessary loss of revenue.